Op so glad you posted this. I am going this weekend too. My fiance is going on business and I must know where to shop. I am hoping I can find some stores that are not in every mall across every city already.
I agree with Soho - if you walk along Broadway in the Soho area, you'll hit Uniqlo, Topshop, Scoop, Madewell (and Wink, Opening Ceremony on the side streets) all in one shot. I think H&M is there as well, but like mielikki55 and nycfashionlvr said, it might not be worth your time. If you like H&M, you should try Topshop.
If the weather's nice, it'll be really fun to walk in and out of the shops in Soho (and West Village too). But if the weather sucks, you might want to go to 5th Avenue instead and check out Barneys, Bergdorfs and Saks as nycfashionlvr suggested. Ooohh, I'm getting excited for you I love NY shopping - have fun!